Options for Wastewater Management

OPTIONS FOR WASTEWATER MANAGEMENT IN HARARE, ZIMBABWE

Options for Wastewater Management in Harare, Zimbabwe DISSERTATION Submitted in fulfilment of the requirements of the Academic Board of Wageningen University and the Academic Board of the UNESCO-IHE Institute for Water Education for the Degree of DOCTOR to be defended in public on Tuesday, 18 May 2004 at 15:00 h in Delft, The Netherlands by INNOCENT NHAPI born in Chegutu District, Zimbabwe Promoter:

Prof. Dr. H.J.Gijzen Professor of Environmental Biotechnology UNESCO-IHE Institute for Water Education, The Netherlands

Co-promoter:

Dr. M.A.Siebel, P.E. Associate Professor in Environmental Biotechnology UNESCO-IHE Institute for Water Education, The Netherlands

Awarding Committee:

Prof. Dr. D.Huisingh University of Tennessee, United States of America Prof. Dr. ir. C.J.N.Buisman Wageningen University, The Netherlands Prof. Dr. P.Marjanovic UNESCO-IHE Institute for Water Education, The Netherlands Prof. Dr. B.E.Marshall University of Zimbabwe, Zimbabwe Prof. Dr. ir. H.H.G.Savenije UNESCO-IHE Institute for Water Education, The Netherlands

Copyright © 2004 Taylor & Francis Group plc, London, UK All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publisher. Although all care is taken to ensure the integrity and quality of this publication and the information herein, no responsibility is assumed by the publishers nor the authors for any damage to property or persons as a result of operation or use of this publication and/or the information contained herein. Published by A.A.Balkema Publishers, a member of Taylor & Francis Group plc. mailto:www.balkema.nlandwww.tandf.co.uk This edition published in the Taylor & Francis e-Library, 2005. To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to http://www.ebookstore.tandf.co.uk/. ISBN 0-203-02711-6 Master e-book ISBN

ISBN - (Adobe e-Reader Format) ISBN 90 5809 697 1 (Print Edition) (Taylor & Francis Group) ISBN 90 5804 044 2 (Print Edition) (Wageningen University)

To my father, who died in the final stages of the preparation of this thesis on 24/06/03: Paidamoyo

CONTENTS Page Acknowledgements Abstract Chapter 1 Introduction Chapter 2 Inventory of Existing Water Management Practices in Harare, Zimbabwe Chapter 3 Impact of Urbanisation on Water Quality of Lake Chivero, Zimbabwe Chapter 4 A Strategic Framework for Managing Wastewater: A Case Study of Harare, Zimbabwe Chapter 5 Options for Onsite Management of Wastewater in Harare, Zimbabwe Chapter 6 Options for Decentralised Management of Wastewater in Harare, Zimbabwe Chapter 7 Centralised Management of Wastewater in Harare, Zimbabwe: Current Practice and Future Options Chapter 8 Effluent Polishing via Pasture Irrigation in Harare, Zimbabwe Chapter 9 Proposal for the Sustainable Management of Wastewater in Harare, Zimbabwe Summary (in English)

viii ix 1 23 40 57 74 94 111 128 142 160

Summary (in Dutch)

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Curriculum Vitae

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ACKNOWLEDGEMENTS The author is very grateful to all those who have generously contributed their time in the production of this thesis. In particular, I feel greatly indebted to my Promoter, Professor H.J. Gijzen of UNESCO-IHE Institute for Water Education, together with my Mentor, Assoc. Professor M.A.Siebel, for their guidance, criticisms and valued advice. Special thanks also to Dr. Pieter van der Zaag and Eng. E.Kaseke for their administrative and supporting roles. I would also like to extend my gratitude to Dr G.G.O’loughlin of Sydney, Australia, for encouragement, and University of Zimbabwe staff from various departments, especially Professors B.E.Marshall and M.F.Zaranyika, and the laboratory staff of the Civil Engineering Department. Part of the work that was carried out and published in conjunction with MSc students—Eng. Misery Mawere and Mrs. Sithabile Tirivarombo—is gratefully acknowledged. Also of special mention is the assistance I received from Eng. Zvikomborero Hoko, Dr. Jerry Ndamba, and Mr. Job Dalu, which resulted in joint publications with each of them. I would like to acknowledge financial support that made my study possible: SAIL Project (Netherlands Government), SIDA/SAREC fund at the University of Zimbabwe, and the Water Research Fund for Southern Africa. I would also like to pay special tribute to all professionals who have contributed to a large body of knowledge in urban wastewater management. The use of their material is acknowledged with thanks. Last, but certainly not least, I would also like to extend my sincere appreciation to my wife and family members for their patience and endurance throughout my studies.

ABSTRACT The capital city of Zimbabwe, Harare, has adopted an urban water cycle that is geared towards high level service provision. Water supply and sewerage/sanitation coverage amounts to over 98%, which makes Harare one of the cities in Africa with the highest coverage. The city’s high volume of water abstraction from its main water resource, Lake Chivero, however, can no longer be sustained. The lake has been seriously polluted by large volumes of (partially) treated effluents from wastewater treatment plants in Harare and the neighbouring town of Chitungwiza. It also receives pollution from agricultural, solid waste, industrial, and natural sources. Most of the wastewater treatment plants in the lake’s catchment are overloaded and they experience frequent breakdowns. This situation has been worsened by repeated years of drought, resulting In the accumulation of nitrogen and phosphorous in the lake. The negative impacts of this have been reflected in periodic fish kills, proliferation of algae and water hyacinth, and the reduction in biological diversity. Other related problems are difficulties in potable water treatment and clogging of irrigation pipes. There is now an urgent need to control pollution loads and to remove contaminants that have accumulated in Lake Chivero over many years. A great deal could be achieved through rational management of the urban water system and the associated nutrient cycle. This should be based on an integrated approach that includes reduction of water consumption, and the wise use of water through pollution prevention/reduction measures. On the water supply side, available options include reduction of water losses (now at±30%), water-saving installations (in households, commerce, and industry), direct reuse (e.g., greywater), and alternative water resources (e.g., rainwater harvesting and groundwater). On the wastewater side, options available include onsite, decentralised and centralised treatment plus reuse. The general objectives of this research were to assess the contribution of wastewater from Harare to the nitrogen and phosphorous inflows into Lake Chivero and, based on this assessment, to formulate feasible sanitary engineering solutions to the problem of excessive nutrient inflows into the lake. The research specifically targeted nutrients because these are the major problem parameters. BOD is largely taken care of via current wastewater treatment and river self-purification processes. The general strategy was to intervene at various levels; i.e., property, decentralised and centralised levels, with various options aimed at reducing water use and limiting wastewater production and reusing or recycling water and nutrients. This strategy would reduce nitrogen and phosphorous flows to the lake, whilst increasing water availability. An extensive water quality and quantity monitoring study in the Chivero catchment was carried out from June 2000 to December 2001 to assess the current situation in terms of water use, treatment and reuse levels, and flow balances. In addition, current contributions of wastewater discharges to nutrient flows in the rivers and Lake Chivero were assessed. Intervention strategies were developed based on an approach, referred to as the “3-Step Strategic Approach” to wastewater management. The steps include: 1)

pollution prevention/reduction at source, 2) treatment in the direction of reuse, and 3) disposal with stimulation of self-purification capacity of the receiving water body. The three steps should be considered in this chronological order. Options considered include source control by the users (residents, industries, etc) using various strategies such as greywater separation and reuse, implementation of toilets with urine separation, and other ecological methods of wastewater management. Other possible options are invoking better behaviour through fees and information, and user responsibility through education, legislative changes and stricter controls over industry. Options for boosting the selfpurification capacity of water bodies include introducing wetlands into the river system via natural overflow, land irrigation, reducing retention time in the lake, etc. Flexible and differential solutions were developed for each landuse category (commercial, industrial and residential). The results of this study confirmed that wastewater plays a major role in the pollution of Lake Chivero. Wastewater contributed over 50% of the annual water flows in the major inflow rivers of Marimba and Mukuvisi. Water quality was found to be an urgent problem that requires immediate action whilst water scarcity was considered a mediumterm problem. The river water quality for points upstream and downstream of wastewater discharge points were far above the 0.03 mg/l TP required for avoiding excessive plant growth in rivers. The lake nutrient concentrations were 2.0+1.3 mg/l TN and 0.6+3 mg/l TP (±standard deviation), reflecting a hypertrophic status. Nearly 70% of the annual phosphorus inflows were retained within the lake, which had a hydraulic retention time of 1.6+1.1 years based on rainfall years 1981/2 to 2000/1. However, for the monitoring period, the hydraulic retention time reduced to 0.4 years due to the heavy rains received in that period. Besides the need to substantially reduce nutrient inflows in Lake Chivero, adequate water inflows are also essential for the flushing out of nutrients from the lake, especially phosphorus. The continued accumulation of phosphorus in the lake sediments leads to an internal phosphorus cycle, further complicating remedial measures. The effective reduction of nutrient inflows into Lake Chivero hinges on solutions related to wastewater management. It is in this context that the “3-Step Strategic Approach” was applied, focusing on wastewater treatment and reuse options at onsite, decentralised, and centralised levels. An aggregation of these options led to the development of short-term, medium-term, and long-term solutions. It was estimated that significant improvements in the lake water quality (to about 0.4 mg/l TN and 0.07 mg/l TP in the medium-term) could be achieved by applying the measures suggested in this dissertation. In addition, the treatment of part of the effluent to tertiary standard and subsequent discharge into Lake Chivero could also reduce the lake hydraulic retention time to below 0.5 years, thereby enhancing the flushing out of nutrients. It was concluded that both water quality and quantity problems in the Chivero catchment could be significantly reduced via improvements in wastewater management in combination with the control of other point and non-point sources of pollution.

Chapter 1 Introduction Parts of this Chapter were submitted as: Nhapi, I. and Gijzen, H.J. (2004) Wastewater Management in Zimbabwe in the Context of Sustainability, paper accepted for publication in the Water Policy Journal, January 2004. Introduction

Developments in Water Management There is now almost universal acceptance of the basic principle that water resources need to be managed in an integrated manner (ICWE, 1992; Cosgrove and Rijsberman, 2000). The relationship between water quality, safe water supply, appropriate sanitation, pollution prevention/control, and the need to feed increasing populations is particularly important in this era of frequent droughts, floods and disease outbreaks. Drought forecasting and mitigation, water security assessment, and water demand management are areas that are receiving attention more than before at both local and international conferences. Related areas are water end-use efficiency, system efficiency, rainwater harvesting, storage and recharge innovations, and reuse strategies. In the 21st century, the philosophy behind water management is most likely going to be driven by the Dublin Principles, the World Water Vision, and the Millennium Development Goals (MDG) (Cosgrove and Rijsberman, 2000; United Nations, 2003). The MDG include commitments to halve, by the year 2015, the proportion of people who are unable to reach or afford safe drinking water and those who do not have access to basic sanitation. Much of the focus will be on efficient drinking water supply, low-cost wastewater management, and effective water quality management as the key to improved water resources management, linking the three to water availability and protein production. The Dublin Principles In early 1992 an International Conference on Water and the Environment (ICWE) held in Dublin, Ireland, adopted what is now called the Dublin Principles. These were presented to the world leaders at the United Nations Conference on Environment and Development (UNCED) in Rio de Janeiro in June 1992. The Dublin Conference proposed concerted action to reverse over-consumption, pollution, and rising threats from drought and floods. The Dublin Statement recognised freshwater as a finite and vulnerable resource, essential to sustain life, development and the environment. The effective management of water resources demands a holistic approach, linking social and economic development with the protection of natural ecosystems. It also recognised that water has an economic value

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in all its competing uses and therefore should be recognised as an economic good. Past failure to recognise the economic value of water has led to wasteful and environmentally damaging uses of water (ICWE, 1992). The potential implications of these recommendations on water conservation (industry, domestic and agriculture) and reuse are very important. Recycling could reduce water consumption of many industries with a potential benefit of reduced pollution. The application of the “polluter pays” principle and realistic water pricing would encourage conservation and reuse. Better water management could significantly increase agriculture, industry and domestic water supplies, significantly defer Investment in costly new water resource development and wastewater treatment facilities, and have an enormous impact on the sustainability of future supplies. World Water Vision In recognition of the unsustainable way water has been managed by society so far, the World Water Council developed the World Water Vision. The idea to develop a Long Term Vision for Water, Life and Environment in the 21st Century—or in short, the World Water Vision—was introduced during the first World Water Forum in Marakech, Morocco, in 1997. The process was co-ordinated by the World Water Council and the results were presented during the second World Water Forum in March 2000 in The Hague (Cosgrove and Rijsberman, 2000). There was no consensus on important conclusions as two groups of opinion, referred to as the globalisation and the antiglobalisation groups, emerged from the conference. However, the two groups supported the Vision21 document, the ‘Water for People’ component, in part, because this document was prepared in a clear bottom-up participatory process. The perception was that the anti-globalisation camp favoured human rights, pluralism and democratic accountability, while those in favour of globalisation supported privatisation, economic valuation of water and the power of the global market. Vision21 Vision21 represents the ‘Water for People’ component in the overall World Water Vision. It is aimed at achieving by 2025 a world in which each person knows the importance of hygiene, and enjoys safe and adequate water and sanitation services. To achieve these ambitious goals, it will be necessary to deal with the shortcomings of the current concept of urban water management (too much, too high or too low quality, too little reuse). The realisation of Vision21 will only be possible through the re-formulation of the current practices and the development of new concepts and approaches for sustainable urban water management. These include the establishment of effective water institutions, water demand management, the development of low water usage facilities and low cost sanitation systems, rainwater harvesting, and the extensive use of resource recovery and re-use approaches for wastewater. A holistic approach is therefore required in which wastewater is seen as a resource, and its management is linked to that of water resources and of nutrients. In fact, resource recovery and re-use approaches could, in addition to water savings, create financial incentives that could be used to cover part of the cost of wastewater treatment.

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The above considerations have been further developed by the Environmental Sanitation Group of the Water Supply and Sanitation Collaborative Council (WSSCC) and have led to the so called ‘Household Centred Approach’ (King, 2000). This and other emerging approaches emphasize the household as the target unit for waste containment methods such as cleaner production techniques, and treatment geared to resource recovery and reuse (Gijzen, 2001). In addition to this, there is a need to analyse the water and nutrient cycles and develop appropriate intervention measures at each stage of the cycles.

Urban water management Urban water management includes abstraction, treatment, storage, transmission and distribution to consumers and the subsequent collection and processing of the generated wastewater. It is now realised that urban wastewater is a vital component of water resources and nutrient management, and that it is crucial for sustainability in the water sector. Here, sustainability amounts to maintaining the abundance and quality of water resources to sustain ecosystems and support future human needs while also meeting current household and commercial/industrial water requirements. An emerging new school of thought believes that the traditional or conventional sanitation approach is very expensive and inefficient in terms of money, resources and energy. Its proponents are calling for radical changes, focusing more on pollution prevention, resource recovery (nutrients, biogas, and water) and reuse (Frijns and Jansen, 1996; Parr, 1996; Jeffrey et al., 1997; Otterpohl et al., 1997; Gijzen, 1998; King, 2000). According to these authors, sustainable solutions could only be found by considering: i. the entire urban water cycle (not focusing just on components); ii. the way in which other material cycles interact with the urban water cycle (N,P,…); iii. that these cycles are closed via shorter loops (e.g. wastewater to be treated to level of water resource before discharge, ammonia nitrogen to be reused in stead of denitrified). Problems of the conventional wastewater management approach A typical traditional wastewater management approach uses “end-of-pipe” and centralised treatment technologies with river discharge of effluent (Fig 1.1). This approach focuses on solving short-term problems instead of avoiding them by using appropriate approaches. It results in unrestricted usage of water, fossil nutrients and energy, thereby suppressing the development of systems with source control. In the current approach, which originates from the 19th century (Gijzen, 1998), water from a nearby water resource is treated to drinking water quality and subsequently used only once for a wide range of uses, many of these not requiring such high quality. Subsequently, and often without treatment, wastewater is discharged back to the same water resource. The effective treatment of wastewater worldwide is very low (estimates suggest below 10% (WHO, 2000)) because of the prohibitive costs of current sewerage and centralised treatment systems. Most of these systems also do not provide a complete solution, as only COD and TSS are targeted, while nutrients and pathogens are usually

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discharged without any form of treatment. As a result, eutrophication and frequent outbreaks of water borne disease are reported worldwide (UNEP, 2000).

Figure 1.1: Linear mass fluxes in the traditional urban sanitation approach (Source: Otterpohl et al., 1998) Some disadvantages of this “mix, dilute, and disperse” conventional sewerage system are: health problems in receiving downstream waters; trace contaminants of dissolved matter; nutrient losses to the atmosphere and the seas; and ignoring responsibilities for maintaining fertile topsoil (Otterpohl et al., 1998). Other problems include high energy demand for the degradation of organic and nitrogen compounds; mixing of different wastewater qualities which reduces reuse opportunities; high failure risk; mobilisation of metals; reduced production of humus (carbon-storing); high operation and rehabilitation costs, and negatively affecting the water quality of downstream water users. The traditional approach to wastewater management mainly focuses on the control of waterborne diseases and preventing the degradation of the urban environment and surface waters. This approach is characterised by high water consumption and large central treatment works. The design of treatment facilities is normally based on the removal of suspended solids and biological treatment with hydraulics largely determining the sizing of holding tanks (Khouri et al., 1994). Water consumption, therefore, has a huge bearing on capital costs. Traditional sanitation systems also require highly skilled labour, imported spare parts, consume a lot of energy and are costly to maintain (Veenstra et al., 1997). Plants are normally built larger than required to allow for upsets and uncertainties in their ability to treat the water adequately. The Irrational use of water has serious implications on wastewater treatment. Water of drinking quality is used for non-potable uses, including the transportation of waste (flush toilets), gardening, cleaning floors, and washing cars. One may wonder why 150–300 litres of potable water per capita is used when only 1–2 litres per day are needed for drinking. is it sensible to dilute the compact volumes of human excreta with large quantities of water simply to transport this waste to another location? The term “drinking

Introduction

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water” seems misleading as more than 99% of the product carrying this name is for purposes other than drinking. Water is erroneously assumed to be an infinite resource when, in fact, it is limited in terms of quality and location. The use of one litre of water will result in an almost equal volume of wastewater, and one litre of wastewater is generally more expensive to treat than a similar volume of drinking water (Gunnerson and French, 1996; Gijzen, 2001). As a result, attempts to copy the expensive and inefficient urban water cycle as used in ‘western’ countries, have resulted in a large proportion of the population in developing regions being left without safe water supply and sanitation services. Not only is the current urban water cycle not sustainable, but it is also coupled to nonsustainable nutrient cycles. For example, the current anthropogenic influx of nitrogen into the biosphere via fertiliser production amounts to some 37% of natural nitrogen fixation (Gijzen and Mulder, 2001). Balancing the N-cycle would require the large-scale introduction of biological nitrogen removal, which, under current technical and economic conditions, is only feasible for a small fraction of the world wastewater production. Probably less than 2% of all nitrogen in wastewater is treated for N-removal (denitrification) and this will lead to a further built up of nitrogen in the environment and in water resources resulting in widespread eutrophication and other problems. It is therefore irrational to spend energy and money fixing nitrogen, and to spend energy and money again for denitrification. The same applies to phosphorus, which is essential for agricultural production. Mineral reserves of phosphorus will last for only about 100–150 years (Otterpohl et al., 1996), and for this reason, it should not be wasted to receiving waters, where it will end up in sediments. A solution to this is reuse and short cycling of nitrogen and phosphorus by coupling water and nutrient cycles with resource recovery and reuse. Methane is another valuable by-product of wastewater. When organic matter is treated anaerobically about 375 I of methane gas can be expected from each kilogram of BOD digested material. Assuming an almost complete conversion of organic matter into biogas, a daily production of 25–45 I of methane per capita can be expected (Gijzen, 1998). The gas could be used as energy to power treatment plants or for pumping wastewater to irrigation areas. A new approach It is obvious that the current approach adopted for urban water does not provide sustainable solutions for the majority of the world population, notably in developing regions. There is therefore a strong case for designers and policymakers to challenge the conventional approach and explore alternatives before committing themselves to a central, mechanised wastewater system. These alternatives should focus on basic needs for dignity and quality of life and should balance these with the needs of the environment. The resource value of wastewater should be recognised and its management should be holistic and form part of an integrated process of managing water resources, nutrient flows and wastes. This requires rationalisation of the food and water cycles to avoid a net accumulation of nutrients and other pollutants in water bodies. The water and food cycles, indeed, are intimately interconnected but they could be improved further via shorter cycles. This entails a cyclical (loop) approach to water and nutrient management

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(Fig 1.2) as opposed to the conventional, linear flow (Fig 1.1). Thus, problems are avoided instead of wasting resources in solving them.

Figure 1.2: Mass fluxes in a sustainable urban sanitation approach (Source: Otterpohl et al., 1998) Wastewater management in Zimbabwe Zimbabwe is one of the few African countries that have been able to provide water and sanitation to over 90% of its urban population (WHO/UNICEF, 2001). By law, all households are compelled to have an acceptable sanitation system before an occupation certificate is issued (Urban Councils Act, Chapter 29.15; Regional Town and Country Planning Act, Chapter 29.6). Onsite systems like bucket systems and pit latrines are not allowed in urban areas of Zimbabwe (Taylor and Mudege, 1997) and only flushing toilets with either septic tanks or conventional sewerage are permitted. Smaller urban centres use mostly septic tanks but few towns have vacuum tankers to periodically empty them. It is estimated that more than 92% of urban households are connected to the sewerage system (Table 1.1). Urban councils are the water authorities in Zimbabwe, and this includes responsibilities for sanitation services according to the Urban Councils Act. Recent strict effluent treatment standards (Government of Zimbabwe, Statutory Instrument 274 of 2000) have increased financial pressures on councils by prescribing tougher effluent standards and high penalties for non-compliance; a situation that encourages tertiary treatment instead of reuse. All effluent and waste discharges require a permit. This has led to increased tariffs for users who are already faced with an ailing economy, abnormally high inflation, escalating prices and soaring unemployment rates. Problems

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relating to serious water pollution are more pronounced in Harare, the capital city, located in the Lake Chivero catchment area (Thornton and Nduku, 1982; Moyo, 1997).

Table 1.1: Number of Zimbabwean urban households using each sanitation technology by Province (Source: Taylor and Mudege, 1997) Province

TECHNOLOGY Flush

Blair

Pit Bucket None No No. of data Households 1.48 1.66 0 0.25 0.01 43,587 7.73 4.97 0.15 1.74 0.04 15,684

Manicaland 96.6 Mashonaland 85.37 Central Mashonaland 87.94 4.88 3.69 0 3.14 0.05 15,160 east Mashonaland 90.43 1.9 4.8 0 2.87 0 60,767 West Matabeleland 91.44 1.33 5.53 0 1.64 0.01 16,719 North Matabeleland 88.37 3.86 5.08 0.08 2.57 0.03 11,964 South Midlands 94.4 1.37 2.23 0 2.01 0.01 70,196 Masvingo 97.01 1.04 0.93 0 1.03 0 24,181 Harare 93.94 1.71 4.19 0 0.13 0.04 359,216 Bulawayo 98.41 0.51 0.65 0 0.42 0.01 145,962 National 92.4 2.6 3.4 0.0 1.6 0.0 average Total 720,450 12,536 24,302 33 5,924 189 763,436 #households *Blair toilets are commonly referred to as Ventilated Improved Pit (VIP) latrines.

(Waste)water management and pollution in the Chivero Basin The location of Harare in Zimbabwe is shown in Fig 1.3 and that of Lake Chivero relative to Harare is shown in Fig 1.4. Lake Chivero has a storage capacity of 247,181,000 m3, a surface area of 26.3 km2, maximum depth of 27.43 m, and an average depth of 9.3 m (Thornton and Nduku, 1982; JICA, 1996; Marshall, 1997;). The lake was created in 1952 and is located 35 km south-west and downstream of Harare. It is the major water source for the city (providing over 70% of its needs) and the neighbouring towns of Epworth, Norton, Chitungwiza and Ruwa. The mean annual rainfall is 830 mm (JICA, 1996; Luxemburg, 1996) and the mean annual runoff is about 140 mm (Department of Water Development, 1995).

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Figure 1.3: Map showing the location of Harare in Zimbabwe (Source: http://www.lib.utexas.edu/maps/cia02/zimbabwe_sm02.gif ) The preliminary results of the 2002 census (CSO, 2002) estimated that the population in the Harare metropolitan area was about 1,900,000. Water consumption in Harare averages 430,000 m3/d and 304,000 m3/d (or 70% of it) is collected as wastewater. Only 23,800 m3/d of wastewater is treated onsite and two main wastewater treatment plants (WTP) serve the city. These are the Firle (capacity 144,000 m3/d) and the Crowborough (54,000 m3/d) wastewater treatment plants. Treatment is mainly by trickling filters (TF) and activated sludge systems incorporating Biological Nutrient Removal (BNR). About 70% of this effluent is reused for pasture irrigation whilst the rest is discharged into the Mukuvisi and Marimba Rivers (JICA, 1996). There are also two waste stabilisation pond systems in Marlborough and Donnybrook with combined treatment capacity of 7,500 m3/d and a new extended aeration plant in Hatcliffe with a capacity of 2,500 m3/d. Overloading and maintenance problems plague all the plants and the quality of the final effluent flowing into rivers is very poor.

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Figure 1.4: Map of study area showing the location of Lake Chivero in relation to Harare and Chitungwiza Because of the poor quality of the WTP effluent discharges, runoff and seepage intrusions from pasture irrigation, and other upstream point and non-point sources of pollution, Lake Chivero is heavily polluted. Previous research focusing on the lake found it to be eutrophic (Robarts and Southall, 1977; Thornton and Nduku, 1982; JICA, 1996; Magadza, 1997; Marshall, 1997). Many of these researchers report that wastewater is the major problem. Research focusing on Mukuvisi River (Zaranyika, 1997; Moyo and Worster, 1997; Machena, 1997; Kamudyariwa, 2000) revealed numerous sources and causes of pollution like industrial discharges, solid waste dumps and WTP effluent. In studies on the Marimba River by JICA (1996), Mathuthu et al., (1997), and Manjonjo (1999), heavy metals and nutrients in the river frequently exceeded WHO limits. It has been recommended that industrial effluents and wastewater discharges be tightly controlled if water quality in the rivers and Lake Chivero is to be managed effectively (Moyo, 1997; Bethune and Roberts, 1999). In 1996 the mean concentrations of total nitrogen (TN) and total phosphorus (TP) in Lake Chivero were 0.51 and 0.27 mg/l respectively (JICA, 1996). Two decades ago, nitrogen and phosphorous were identified as the main nutrients limiting phytoplankton growth (Robarts and Southall, 1977; Watts, 1982). In the past few years, a number of planned developments such as the expansion of Crowborough and Firle wastewater treatment plants have failed to take place. A harsh economic environment, characterised by lack of foreign currency and electric power cuts also affects the proper functioning of these plants.

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Lake Chivero under a “business as usual” scenario The water quality problems in the Chivero catchment are likely to be exacerbated by rapid industrialisation and high population growth rates in urban areas (Zanamwe, 1997; CSO, 2002). Using the annual average inter-censal (1992–2002) growth rate of 2.5% (CSO, 2002), the population in the Chivero catchment area is expected to increase to approximately 2,050,000 by 2005, and to 2,620,000 by 2015. This will lead to increased water consumption, resulting in larger water abstractions from Lake Chivero. It will also result in an increased volume of wastewater discharges, whilst natural river flows will increase marginally because of an increase in impervious areas caused by urban construction. The current rainfall trend suggests more frequent drought years (Luxemburg, 1996) and this means that rivers would increasingly be carrying more WTP effluent than natural flows. Lake spillages would decrease with the lake increasingly becoming a major pollutant sink, leading to serious water quality problems. Figure 1.5, derived from the 1996 monitoring data from JICA, shows that there was already a net accumulation of phosphorous and nitrogen in the lake, resulting in a stressed system that needs urgent action.

Figure 1.5: Nitrogen and phosphorous fluxes and concentrations in Lake Chivero for 1996 (derived from JICA (1996) and Zimbabwe National Water Authority (ZINWA) database) Increased wastewater discharges will reduce hydraulic retention times and the selfpurification capacities of receiving rivers (Machena, 1997) impacting on water quality in the rivers. The major wastewater treatment plants (Crowborough and Firle) are close to the Lake, so there will be insufficient time for natural purification to play a significant role in the current set up. The water balance of Lake Chivero (Fig. 1.6) is affected by periodic droughts when flows over the spillway of the dam are very low (or do not occur) and the accumulation of pollutants in the lake increases.

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Figure 1.6: The 1996 annual water budget for Lake Chivero, excluding groundwater flows; flows expressed as 106 m3/yr (Derived from JICA (1996) and ZINWA database) The hydraulic retention time of 403 days in Lake Chivero (Fig 1.6) is not adequate for the flushing out of phosphorus, a process that is more effective in lakes with retention times of less than six months (Thornton, 1980). The water quality problems of excessive algae and water hyacinth (Eichhornia crassispes) growth, clogging of irrigation pipes, impaired boating and fishing activities, occasional fish kills, and water treatment difficulties (frequent filter backwash, odours, taste) are likely to worsen if no action is taken. The City of Harare will face increased problems in ensuring safe and sufficient water supply especially in the dry season when urban water demand, water evaporation from the lake surface, and lake productivity (hyacinth and algae blooms) is at its highest. The disposal of effluent and sludge on pastures in Harare is already showing limitations in removing nutrients and pollutants such as heavy metals (McKendrick, 1982; Nyamangara and Mzezewa, 1999; Manjonjo, 1999). Sustainable nutrient removal via pasture irrigation requires more land, a resource that is now becoming both scarce and too costly for this purpose Furthermore, the efficiency of the land treatment of effluent decreases during wet weather (McKendrick, 1982). Urban agriculture, and its consequences on water quality is also likely to continue unhindered into the future (Mbiba, 1995; Zanamwe, 1997), with chemical and fertiliser applications probably higher than in normal farming (ENDA-Zimbabwe, 1996). The cumulative loading of chemicals and fertilisers on the soils and groundwater, and subsequent leaching into the lake, will be a problem in the future but urban agriculture could have positive effects by reusing nutrients and water if organised differently. If no intervention measures are put in place, the present problems related to effluent disposal and downstream pollution in Harare will continue and increase in magnitude. The lake will become more eutrophic and the quality and quantity of urban water supplies will be adversely affected as will other uses of the lake, e.g. recreation and fishing. This research project was therefore motivated by the need to seek effective ways of managing water resources and pollution in the Harare sub-catchment area of Lake Chivero. Emphasis is put on wastewater management, especially the control of water, nitrogen and

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phosphorus flows. The philosophy driving the research dictates smallest cycles for nutrients and water (not only the end of the pipe).

Specific research problem The main water management problem in Harare is that wastewater discharges contribute significantly to eutrophication in Lake Chivero, although the current extent is not known. The problem is compounded by the fact that water release from the lake does not takes place in years of low rainfall as the dam’s floodgates are permanently closed. Spillway discharges normally take place only from January to April, meaning that the lake acts as a sink for pollutants for most of the year. As the population increases, the lake will increasingly receive a higher fraction of WTP effluent whilst raw water abstraction will also increase, posing a water quality and quantity problem. Nutrient concentrations in the lake are higher than the allowable limits of